﻿<p>An <em>IfcExternalReference</em> is the identification of information that is not explicitly represented in the current model or in the project database (as an implementation of the current model). Such information may be contained in classifications, documents or libraries. The <em>IfcExternalReference</em> identifies a particular item, such as a
dictionary entry, a classification notation, or a document reference within the external source.</p>

<p>Only the <em>Location</em> (as a URL) is given to describe the place where the information can be found. Also an optional
<em>Identification</em> as a key to allow more specific references (as to sections or tables) is provided. The <em>Identification</em> defines a system interpretable method to identify the relevant part of information at the source. In addition a human interpretable <em>Name</em> can be assigned to identify the information subject, such as a classification code.</p>

<p><em>IfcExternalReference</em> is an abstract supertype of all external reference entities.</p>

<blockquote class="history">
HISTORY&nbsp; New entity in IFC2x.
</blockquote>

<blockquote class="change-ifc2x4">
IFC 2x4 CHANGE&nbsp; Attribute <em>Identification</em> renamed from ItemReference, attribute <em>Location</em> datatype changed, and inverse attribute <em>ExternalReferenceForResources</em> added.
</blockquote>